1. Field of the Invention
The present invention relates to UV-curable binders and their use for the preparation of high-gloss coatings which can be cured by free-radical polymerization initiated by UV light.
2. Description of the Prior Art
It is known that unsaturated polyester resins, in particular those dissolved in styrene, are suitable for the production of high-gloss coatings on rough or porous surfaces, such as wood. A distinction is made here between paraffin-containing, and air-drying (also called gloss polyesters) types (Glasurit-Handbuch Lacke und Farben. Curt R. Vincentz Verlag, Hannover, 11th edition, 1984). Paraffin-containing unsaturated polyesters are applied in thick layers of up to approx. 1 mm and after curing initiated with (hydro)peroxides and metal siccatives, are usually sanded and polished to a high gloss. Air-drying unsaturated polyesters can cure to a high gloss by themselves, but have a lower reactivity than the paraffin-containing products and usually cannot be readily polished. Air-drying unsaturated polyester resins can also be cured under UV light using photoinitiators. Although this increases the rate of curing, it is difficult to polish the resulting surfaces to a high loss or for repair purposes.
Substantially higher reactivities than with unsaturated polyesters can be achieved by curing unsaturated resins based on acrylate with ultraviolet radiation (according to Glasurit Handbuch Lacke und Farben, reactivity difference factor of 4 to 5. according to W. Fischer, I-Lack 61, 2, 1993, 54 to 58, reactivity difference factor of up to 35). However, compared with unsaturated polyesters, unsaturated acrylates have the disadvantage that it is difficult to cure them in a stress-free form in thicker layers and they cannot be sanded satisfactorily and above all have very poor polishability. Attempts have been made to improve the polishability of these coatings by addition of elastic resins. However, on absorbent or uneven substrates, such as wood, it has been observed that the coatings have very poor adhesion.
An object of the present invention is to provide UV-curable binders, which are suitable for the preparation of high-gloss coatings and do not have the disadvantages of the prior art.
This object can be achieved with the binders according to the invention, which are distinguished by a high reactivity during curing. Coatings produced therefrom have good polishability and stability and form high-gloss surfaces.